The present invention is directed to a method of producing cellulose derivatives using cellulose modified by crosslinking and to the resulting products. The method is particularly directed to the use of wood celluloses that might not otherwise be suitable for production of the particular derivative.
Cellulose derivatives have been commercially available since the advent of camphor plasticized nitrocellulose as the first synthetic plastic material in 1868. About the time of the first World War cellulose acetate began to be used as a nonflammable aircraft lacquer. It was not until the decade of the 1940s that cellulose ethers became commercially important. Water soluble methyl cellulose was introduced in 1939 and ethyl cellulose became important during World War II. Sodium carboxymethyl cellulose (NaCMC) became available shortly after the war and today is one of the most widely used cellulose derivatives. Many other cellulose ethers are produced as specialty chemicals.
For many years purified cotton linters was the major raw material for production of cellulose esters and ethers. However, for some products high alpha cellulose sulfite or prehydrolyzed kraft wood pulps have also been used. If NaCMC may be used as an example, it is available in various viscosity grades. The term xe2x80x9cviscosityxe2x80x9d here refers to viscosity of a water solution. In turn, solution viscosity is closely related to degree of polymerization (D.P.) of the cellulose raw material. High viscosity grades have traditionally been produced from cotton linters cellulose while medium and low viscosity grades are made using lower cost wood pulps. The D.P. of most wood pulps does not exceed about 1500 although some specialty dissolving pulps have considerably higher D.P. Cotton linters cellulose can be readily produced with a D.P. of 2400 or greater.
Because of their cost advantage, it would be advantageous to the manufacturers of various cellulose derivatives if wood pulps could be used more extensively. Although not limited to wood derived cellulose sources, the present invention serves that end by increasing the effective D.P. of celluloses to make them suitable raw materials for derivatives requiring a high D.P starting material.
The present invention relates to preparation of cellulose derivatives using crosslinked cellulose as a basic raw material. By introducing a controlled amount of crosslinking, the effective D.P. of the cellulose may be significantly raised without destroying cellulose reactivity. In turn, the crosslinked product may be used for preparation of derivatives that formerly required very high D.P., and usually very high priced, cellulose sources as a starting raw material.
The preferred cellulose is a wood pulp cellulose of the type normally referred to as a xe2x80x9cdissolving pulpxe2x80x9d or xe2x80x9chigh alpha chemical pulpxe2x80x9d. These are cellulosic materials normally prepared by the sulfite or prehydrolyzed kraft processes. Other cellulose sources having sufficient reactivity with etherification or esterification chemicals after crosslinking are also considered suitable. Among these are paper grade sulfite pulps, the paper or fluff grades of kraft pulps nominally called xe2x80x9ckraft market pulpsxe2x80x9d, cotton linters cellulose, and cellulose obtained from other plant or bacterial sources.
The term cellulose derivatives should be read broadly and is intended to include both esters, such as nitrocellulose and cellulose acetate, and ethers such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, carboxyethyl cellulose, carboxypropyl cellulose, hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, and many others.
Crosslinked celluloses of various types are well known; e.g., see G. C. Tesoro, Cross-linking of cellulosics, In Handbook of Fiber Science and Technology, Vol. II, M. Lewin and S. B. Sello eds., pp. 1-46, Marcel Decker, New York (1993), as exemplary. Crosslinking forms chemical or ionic bridges between hydroxyl groups on adjacent cellulose molecules. Commercially, crosslinking is important in cotton fabric finishes to provide wrinkle resistance. Crosslinked wood pulp fibers are also used in other consumer articles such as disposable diapers or paper towels where their relative stiffness serves to increase bulk and compression resistance. However, to the present inventors"" knowledge, crosslinked cellulose has never before been used as a raw material for production of cellulose derivatives.
While many crosslinking agents are believed to be suitable in the present process, those that produce compounds resistant to D.P. loss during derivitization are preferred. In particular, those that form ether linkages with the cellulose are particularly preferred. Agents that produce hemiacetal linkages have also been found to be suitable.
It is an object of the present invention to provide a method for production of various cellulose derivatives using crosslinked cellulose.
It is a further object to provide high effective D.P. wood pulps as a raw material for derivitization.
It is another object to provide cellulose derivatives made using wood pulps which have the D.P. characteristics of cotton celluloses.
It is yet an object to provide cellulose derivatives having entirely new properties heretofore unattainable.
It is also an object to produce a high viscosity grade of NaCMC from a modified low cost wood pulp cellulose.
These and many other objects will become readily apparent upon reading the following detailed description taken in conjunction with the drawings.